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Bugman1400



Joined: 14 Apr 2013
Posts: 8

PostPosted: Sun Apr 14, 2013 8:52 am    Post subject: RCA G35831AT CTC179CM Reply with quote

I have a 1995 36" CRT that tries to turn on but, doesn't. When I press the POWER button, the green LED comes on and I hear three faint clicks and then the unit shuts down.

Any ideas?

I am also an EE and have access to a Fluke 87 and a 60MHz O-scope.

Mucho thanks!
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curtis wolfe



Joined: 11 Apr 2013
Posts: 27
Location: connecticut

PostPosted: Sun Apr 14, 2013 12:45 pm    Post subject: Reply with quote

DEAD (CYCLES 3 TIMES HIGH VOLTAGE COMES UP TO 20KV THEN SHUTS DOWN)

REPLACE T4401 HIGH VOLTAGE TRANSFORMER (PN: 227271 SAME FLYBACK USED FOR DUAL FOCUS MODELS (PN: 227318) USED IN CTC179DL MODEL F35760MB) (NOTE: FOR SIGNAL FOCUS MODELS YOU MUST REUSE HGH VOLTAGE ANODE CAP & CUT OFF FOCUS PLUG & WIRE DIRECTLY INTO SCREEN/FOCUS CONTROL ASSEMBLY)
Bugman1400



Joined: 14 Apr 2013
Posts: 8

PostPosted: Sun Apr 14, 2013 1:09 pm    Post subject: Reply with quote

curtis wolfe wrote:
DEAD (CYCLES 3 TIMES HIGH VOLTAGE COMES UP TO 20KV THEN SHUTS DOWN)

REPLACE T4401 HIGH VOLTAGE TRANSFORMER (PN: 227271 SAME FLYBACK USED FOR DUAL FOCUS MODELS (PN: 227318) USED IN CTC179DL MODEL F35760MB) (NOTE: FOR SIGNAL FOCUS MODELS YOU MUST REUSE HGH VOLTAGE ANODE CAP & CUT OFF FOCUS PLUG & WIRE DIRECTLY INTO SCREEN/FOCUS CONTROL ASSEMBLY)


I bought the Thompson's Service Manual for the CTC179 for giggles and since it was only $18 and available for download. The Service Manual talkes about the Overcurrent Protection circuit and is the reason for the 1/2 second interval clicking I hear. I also read the Bulletin that indicated a similar symptom had a faulty flyback (T4401). However, the Overcurrent Protection circuit senses the primary of T4101 (Power Supply Transformer). There are several secondaries, one being the 135V source to the horizontal deflection circuit (flyback xmfr). However, couldn't an overload on any of the secondaries cause an overload on the primary of T4101? Also, since the flyback is a xfmr can't you test for a failure mode? I would expect that I could test for shorted windings or a short to the iron on both the primary and the secondaries. Furthermore, It seems that if I desolder the flyback and re-energize the unit, then it should remain on!

Thoughts?
JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Sun Apr 14, 2013 4:18 pm    Post subject: Reply with quote

FREE (Sams Photofact)

http://elektrotanya.com/rca_ctc179cm_chassis.pdf/download.html
Bugman1400



Joined: 14 Apr 2013
Posts: 8

PostPosted: Sun Apr 14, 2013 6:00 pm    Post subject: Reply with quote

JTS1957 wrote:
FREE (Sams Photofact)

http://elektrotanya.com/rca_ctc179cm_chassis.pdf/download.html


Thanks. It doesn't describe the circuit functions but, it does give a nice variation on the schematics.

I am still trying to figure out what turns the flyback xfmr on when the POWER button is pressed. It appears that the main power supply xfmr (T4101) is always on, as long as the TV set is plugged in, since it supplies power to the Infrared control to allow the remote to work. So, there must be something that switches the 135V to the flyback xfmr ON/OFF when you press the POWER button on the TV set or remote.
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JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Sun Apr 14, 2013 8:57 pm    Post subject: Reply with quote

It SHOULD already be THERE too (Q4401 collector). In fact on page 1B of Sams, sources 1 thru 9 should ALL be there when set is plugged in (All-Time).
7.6V at U2001 pin #24 should start Horizontal drive signal for high voltage.
Bugman1400



Joined: 14 Apr 2013
Posts: 8

PostPosted: Mon Apr 15, 2013 7:20 am    Post subject: Reply with quote

So, the +135V is always at the flyback (T4401) on pin 8. Its the ground or return that is getting switched ON at pin 10 by Q4401. Is that correct?

Also, if the flyback is not always fully energized and producing the secondary voltages then, is it possible that when the Q4401 switches ON then too much current drawn by that circuit could cause too much current thru T4101 and thus, trigger the overcurrent protection circuit?
JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Mon Apr 15, 2013 4:08 pm    Post subject: Reply with quote

There is NO ground pin in T4401 Primary.

For the flyback to be "energized" it requires a DC voltage source (from power supply) and a "signal" (Waveforms at: Pin #26 of U2001 / Emitter of Q4301 / Collector of Q4302 / and base of Q4401).

Use your "60MHz O-scope" to see if any of these waveforms appear each time you "press the POWER button, the green LED comes on and I hear three faint clicks" on.
Bugman1400



Joined: 14 Apr 2013
Posts: 8

PostPosted: Mon Apr 15, 2013 7:18 pm    Post subject: Reply with quote

JTS1957 wrote:
There is NO ground pin in T4401 Primary.

For the flyback to be "energized" it requires a DC voltage source (from power supply) and a "signal" (Waveforms at: Pin #26 of U2001 / Emitter of Q4301 / Collector of Q4302 / and base of Q4401).

Use your "60MHz O-scope" to see if any of these waveforms appear each time you "press the POWER button, the green LED comes on and I hear three faint clicks" on.


Do you mean the Collector of Q4401? What is the purpose for testing these locations? Is it to establish that these parts are working correctly for the 1/2 second interval? If they are, will I test further down the line and have to desolder T4401?

.......just checking to see if I am getting this.
JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Tue Apr 16, 2013 3:20 am    Post subject: Reply with quote

I "meant" the four points I stated.

BUT ... IF your scope has sufficient protection (say a Sencore SC3100) you "could" directly measure the collector of Q4401. If it doesn't, you could set your scope for approximately 10us/Div (horizontal rate) (Volts/Div to mid-range - will vary with probe position) and bring the PROBE itself "close" to the windings of T4401 (WITHOUT actually "connecting" it to ANY point). Expect a momentary waveform similar in SHAPE to the one shown on the CRT filament, but much larger, with no smaller peak(s) between the normal peaks.

The purpose is to prove if your power supply is attempting to start the set and to see if it's due to a commonly defective flyback as curtis wolfe suggests.
RCA used "three strikes and you're out" circuitry which won't allow the power supply to continue to run if it sees a error in various parameters (over current is one). If the supply fails to stay running after three attempts, it usually requires set to be unplugged to reset.

Quote:
Also, since the flyback is a xfmr can't you test for a failure mode? I would expect that I could test for shorted windings or a short to the iron on both the primary and the secondaries.
Going back a ways ...
What were you planning to use to test it for a failure mode? A Sencore TVA92? A flyback tester? Certainly NOT a standard ohmmeter? While it WOULD find the a short from one winding to another or a winding to the iron core, it wouldn't be able to detect shorted turns within a winding.

Quote:
Furthermore, It seems that if I desolder the flyback and re-energize the unit, then it should remain on!


You next will be floating the collector of Q4401 to see if there is a difference in operation, but the horizontal output section is one of the largest current draws on the power supply and it might sense a under current condition and shut down after three attempts.
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